The present invention relates to a method for manufacturing a plastic case and more particularly to a plastic case for housing a tape casette, floppy disk or other recording medium.
Heretofore, a magnetic tape cassette, a floppy disk cassette, an optical disk cassette or the like have been protected by housing it in a plastic or paper case so as to prevent dust from adhering to the surface of the recording medium in the cassette or to prevent impairment to the cassette which, otherwise, would lead to loss of memory, wrong recording or reproduction.
In such case, when, for example, a sleeve-shaped plastic case is to be formed from a blank, a sheet manufacturer produces rectangular cut sheets of a plastic material and ships the sheets to a case manufacturer. The case manufacturer, then, prints a plurality of designs and other necessary indications in some areas on one surface of the sheet corresponding to respective blanks to be cut out of the sheet. Next, the sheet is placed on an anvil plate of one of a pair of cutting dies and is cut with the other die having a cutting blade into a plurality of blanks of the same size at a time. During this process, the outer contour of each blank having a contour of an unfolded plastic case is cut along the outer ontour and separated from the sheet. The resulting blanks are each placed on an anvil plate of one of scoring dies and scored by hot press using a scoring blade of the other die while heating the blade and the anvil plate to form necessary scores or grooves for folding the blank into a case. Each of the resulting scored blanks is put in a case-forming machine in which the blank is folded up and welded together at predetermined locations of superposed portions, thereby obtaining a plastic case.
However, with the plastic case manufacturer, although a number of blanks can be cut out from a large sheet, it is not easy to perform the subsequent scoring of the blanks in the same continuous production line because the scoring of blanks one after another demands a burden in time and labor, resulting in inefficiency and high cost of the product. The present applicants proposed in Japanese Patent Application No. 4-78567 a process in which a starting molten plastic material is molded by passing the material through a nip between a pair of cylinders which simultaneously form a sheet and score folding grooves on the sheet corresponding to a plurality of blanks. The resulting sheets are shipped to a case manufacturer where each of the sheets are cut into a plurality of blanks at one time.
However, this is still cumbersome because the sheet manufacturer must ship sheets having folding scores to a case manufacturer who, in turn, cuts the blanks with cutting dies to obtain finished cases. Accordingly, plural steps still remain and the cost is not sufficiently saved. Further, the residue of the sheet must be sent back to the sheet manufacturer after the blanks are cut off from the sheets for recycling purpose. It is not rare that the residue is discarded as a trash from the economical standpoint. In addition, the dimensional precision of the products is not very high because the precise registration of the contour of the blank of the sheet with the cutting dies is not easy to establish.
The present invention aims at providing a method for providing a plastic case for recording medium or the like with high efficiency and precision at a low cost.